The civil war in the labor movement that led to this week's split in the AFL-CIO is the latest skirmish in a longer struggle. The same forces that helped make John Sweeney president of the union federation in the first place have led the dissenting unions to denounce him and take a walk.

The movement's troubles are rooted in frustration over an environment that is increasingly hostile to organized labor, an economy that has hemorrhaged the sorts of industrial jobs unions organize best, and a community of employers increasingly willing to resist unionization.

These facts led to Sweeney's election as an insurgent a decade ago. In defeating his friend Tom Donahue, the AFL-CIO's acting president, Sweeney promised much of what rebel leader Andy Stern now proposes, notably increased spending on organizing and a tougher response to a tougher environment.

In 1995, Sweeney asked the support of all who were "tired of being treated like so much road kill on the highway of American life." And in a line now freighted with irony, Sweeney declared: "As your president, I will never forget that our movement grows by addition and multiplication, and not by division and subtraction."

The ironies keep adding up: Sweeney was president of the Service Employees International, the same union Stern has just led out of the labor federation along with the Teamsters. Stern's cachet now, like Sweeney's then, is based in large part on the SEIU's success: It invested more than most unions in organizing, and it grew fast. Ten years ago, Sweeney was the new guard replacing the old. Today, Stern is the insurrectionist against Sweeney's establishment.

By pulling out of the AFL-CIO and proposing to start an organization, Stern and his Teamster ally James Hoffa -- other unions may join them -- are embarking on a laboratory test. "Our world has changed, our economy has changed, employers have changed," Stern has said. "But the AFL-CIO is not willing to make fundamental change."

The test is over which half of that statement is more important. If they succeed, Stern and Hoffa will show that labor's decline has been primarily the result of flawed strategy and tactics, which can be corrected. If they fail, the dissidents will become the victims of the same forces that have led to Sweeney's troubles.

The tragedy of labor is underscored by the reaction of its friends: There has been a great reluctance to take sides. Few know whether the split will invigorate the labor movement or cripple it. Few sense an obvious balance of right and wrong. The well-liked Sweeney represents loyalty and solidarity. Stern and Hoffa stand for impatience, determination and boldness. Labor needs all these virtues.

Rep. Rahm Emanuel of Illinois, the chairman of the Democratic Congressional Campaign Committee, said that the split is a long-term gamble for the labor movement itself, but he's worried about its immediate impact on Democrats. "Will the division have an effect on the election of 2006?" he asks. "We'll know that pretty quickly."

Whatever else Sweeney failed to do, he clearly built an effective political organization. In 2004, John Kerry carried Pennsylvania by defeating George Bush among voters living in union households (30 percent of the electorate) by 62 to 37 percent. Kerry lost the rest of the voters here, 55 to 45. In Michigan, Kerry won 61 to 37 in union households (37 percent of the electorate) and lost the rest, 55 to 44 percent.

The problem, as Stern would insist, is that even in Pennsylvania, labor's share of the workforce has been cut by more than half over the past four decades. In the South and Southwest, the movement is even more enfeebled. Thus the chicken-and-egg question at the heart of this struggle: Labor needs to have a strong voice in politics to be successful, and it needs to be successful to have a strong voice in politics.

This split might at least remind liberals and Democrats of how much they have depended on unions. Democrats have treated organized labor as a cross between an ATM and a temp service: Politicians would show up to grab money and people at campaign time, but they did little to maintain the machine and the workforce after Election Day. As the political philosopher Joni Mitchell put it, "You don't know what you got till it's gone."

It's too late to avert the split, but not too late to help revive a movement that has been essential to achieving social justice in the United States. For liberals, the Sweeney-Stern confrontation underscores the urgency of standing up for labor at its moment of crisis.